Freshly distilled methylene diphenyl diisocyanate isomer mixture which contain a high concentration of the 2,4'-isomer (2,4'-rich MDI) and prepolymers based thereon are generally colorless. These materials undergo significant discoloration upon storage even in the absence of light. They tend to yellow and occasionally turn brown. While discolored 2,4'-rich MDI can be distilled to prepare a colorless product that is only a temporary measure. The discolored prepolymers can not be distilled to prepare a colorless mixture.
The stabilizer of the invention appears to have a synergistic effect in maintaining the color of prepolymers based on 2,4'-rich MDI. The prior art as represented by U.S. Pat. No. 2,885,420 disclosed color improvement of monomeric isocyanates resulting upon the addition of ethers or thioethers including phenyl glycidyl ether. The aging conditions however demonstrated in the reference were relatively mild (storage at room temperature for 100 hours at 20% relative humidity). In prepolymers based on 2,4'-rich MDI the addition of an epoxide alone is not effective as a stabilizer upon aging at higher temperatures and at longer aging times.